Product(s):	WaterCAD, WaterGEMS
Version(s):	08.11.XX.XX and higher
Area:	Calculations

Problem

When computing a model, the following user notification is generated:

"Disconnected demand nodes detected; hydraulic results are likely to be compromised."

Solution

This user notification means that there are junctions in the model that have a demand applied to them but are physically disconnected from a source, such as a tank or reservoir. For example due to closed pipes or valves. When there are disconnected demand nodes, this will lead to issues with the hydraulic results and the user should consider them invalid or requiring close scrutiny.

Note: Starting with WaterGEMS and WaterCAD 2023 (version 23.00.00.16 and higher), if a demand is disconnected, the program automatically makes the demand zero, so that no flow is forced through closed elements. The user will now see a Yellow Disconnected Node User Notification rather than a Red one. Regardless of the version, the hydraulic results may be compromised.

However, the pressure in the disconnected network may not necessarily be zero. In some situations a non-zero and seemingly random pressure / HGL will be seen in the disconnected network. Potential improvements to this are being tracked in reference number 1561470. As a workaround you can set the calculation type to a SCADA run; this should zero out calculated results for all disconnected nodes.

There are many possible causes of a disconnected network. For instance, a pump or valve that is closed will stop flow coming from upstream and physically disconnect parts of the model, such that the HGL cannot be determined. If there is no other potential source of flow in the system, the demands will not be satisfied.

Other areas that can be checked include tank settings. If a tank is empty, it will automatically close the adjacent pipe and will not be able to supply flow. You can read more about what happens when a tank becomes empty or full here.

Make sure that the orientation of your pumps and/or valves is correct and that the pumps' "Status (Initial)" is set to "On" (or have controls to turn them on). If you have the incorrect "downstream link" selected (particularly for a pumps) this will inhibit flow from the element.

If you have a hydropneumatic tank in the model as a source, make sure the property field "Treat as Junction" is set to False. If it is set to True, the hydropneumatic tank will act like a junction instead of a potential source of flow.

Another item to check is the status and connectivity of pipes in the system. If a pipe is set to be closed, it will cut off flow from upstream. It is also possible that a pipe is not connected to the rest of the system. For instance, a pipe may be overlaying another pipe rather than be connected to it. These types of connectivity issues can be difficult to spot visually, but there is a tool called Network Navigator that can be used to find disconnected segments of a model. See the article on Using Network Navigator for more information.

If you intentionally cut off the demands by closing the valve or pipe and wanted to see the flow drop to zero, you will need to use the Pressure Dependent Demand (PDD) feature. With PDD, if the pressure drops to zero when closing the valve, the demand will drop to zero. Alternatively, you could simply remove the demands in conjunction with closing the pipe (you could use a new scenario with new demand alternative in this case, to preserve the original demands). Also, if you want to look at the effect on the system of closing off isolation valves, consider the Criticality tool.

Note: In earlier versions, the demands will still be reported and you will see flow through closed elements, like valves or pumps.

In this condition, flow will pass through the closed valve. The reason is because when you enter a demand at a node, you are telling the model that it must satisfy that demand no matter what. So, by closing off a demand, those two things are at odds with each other. The demand is still satisfied, flow goes through the closed valve/pipe and you see this notification.

Disconnected Demand Nodes user notification

Problem

Solution

See Also